After a pregnancy test revealed that her final attempt at IVF had failed earlier this year, Suzanne Harper allowed herself a tiny smile through her tears. While she was facing the fact that there would be no baby, she was also aware that someone, somewhere in Britain had recently given birth to twins who were genetically hers, thanks to her donation of eggs. Suzanne is adamant that if only one of them could be successful in their quest for a child, she's glad it was the other woman. "I know the anguish of infertility – and I also know the extraordinary joy of having a baby thanks to someone else's kindness," she says.

Suzanne, 34, and her husband Mark, 35, who live in Derbyshire, already have a daughter Libby, aged two. She was conceived with sperm from an American donor after they discovered that Mark was infertile. It was when she was having IVF, says Suzanne, that she first considered the idea of egg-sharing. The procedure – commonly used at fertility clinics – enables a woman to donate eggs to another couple in return for discounted treatment. "But it wasn't the economics that attracted me – it seemed such an easy thing to do," she says. "Someone else had given his sperm for us, so it seemed a kindness in return to give some of my eggs to someone else."

However, Mark talked her out of it. "I thought, what's it going to feel like for Suzanne if she doesn't have a baby and then another woman gets pregnant using her eggs?" he says.

He felt she had had enough to cope with. When the couple married in 2002, after seven years together, they felt ready to start a family. Suzanne is the kind of women who exudes earth mother qualities. Long before she met Mark she was a nursery nurse, honing skills she knew she wanted to use in her own life one day. "She was born to be a mother," says Mark.

But a year passed without a pregnancy. "We went to the doctor and were referred for fertility tests," says Suzanne. Mark had been treated for testicular cancer a few years before, but had been assured his fertility was unaffected. "The news that I had a zero sperm count came as a massive shock," he says. "We both come from very family-oriented backgrounds: this was what it had all been about. I knew how important children were to Suzanne. I couldn't bear the thought that my problem was going to have such a massive impact on her life. So I said, you're going to have to find someone else to have your babies with …"

"But I'd never have done that!" says Suzanne. "I said to Mark, you're the love of my life. We're in this together."

"At that point," says Mark, "I realised we had to explore sperm donation. It wasn't an easy thing to start thinking about: but if Suzanne was prepared to stick with me, and this was the only way of getting a baby, I owed it to her to come to terms with it."

Two attempts at insemination at an NHS clinic failed, and further tests showed they would have a better chance of success using IVF – though it was more invasive and, as they had decided to use a private clinic, more costly. Two cycles of IVF followed; then four. Suzanne was given immunosuppressant drugs to improve her chances of pregnancy. On cycle six, she conceived, only to miscarry a few days later. On cycle eight, the clinic tried a new immunosuppressant drug, and her eggs were fertilised with sperm from the US. "We'd decided this would be it, because we couldn't keep going for ever," says Suzanne. "If this round didn't work, we planned to explore adoption."

But this time she was pregnant and stayed pregnant. On December 19 2009, Libby Pauline was born, weighing 6lb (2.7kg). Suzanne had been told during treatment that if she managed to have a baby and wanted another, she shouldn't wait long. So when Libby was one she went back to the clinic. This time round, she was determined to share her eggs. "I was so thrilled to have Libby in our lives. I felt this overwhelming urge to do for someone else what someone else had done for us," she says. "I kept thinking, there's a couple out there whose lives I could change for ever."

The process was anonymous – but Suzanne's menstrual cycle had to be synchronised with that of the recipient, so that when the eggs were harvested they could be fertilised and implanted at the right moment. "So all the way through it, I was thinking of her and the fact that she was going through exactly what I was," says Suzanne.

"The clinic told me that, if I didn't produce enough eggs for both of us I could choose to keep them. I knew I couldn't do that to her, though. If I'd only produced enough for one of us, it would have been her.

"I couldn't have made her go through all the drugs and treatment only to hear, on the day of implantation, that there were no eggs after all."

As things turned out, Suzanne produced 10 eggs – five for her, and five for the recipient. "I was over the moon. Also, we were able to use sperm from the donor who was Libby's genetic father, so I was really positive about it all, convinced it was going to work – for both of us, me and the unknown woman who was using my eggs."

Two weeks later, her pregnancy test was negative. "I was very upset but, when I phoned the clinic to tell them, I knew that they'd also know that day whether the recipient was pregnant, because her cycle was the same. So I asked. The nurse went away to check and came back to tell me that she was. The best thing would have been for us both to have been pregnant, but her being pregnant felt like a silver lining. And I had other frozen embryos, so I still had a chance of another baby."

Five months later, another embryo was implanted and this time Suzanne miscarried at six weeks. "Now I had just one embryo left, so I knew I was in the last chance saloon," she says. "Mark and I talked it through and we decided we'd give it our best shot, but if it didn't work out Libby would have to be an only child.

"We couldn't put our lives on hold to keep trying for another baby – and I didn't want her to grow up remembering a mummy who kept having to give herself injections [for the fertility hormones] and having to go to lots of hospital appointments."

Meanwhile, the other woman's pregnancy was progressing. "I thought about her a lot," says Suzanne. "One day when I was at the clinic I asked how she was getting on. They went to look at her notes and said, she's had her scan – it's twins! I burst into tears. I knew she only had that one chance of a family – the other eggs I'd donated hadn't fertilised properly."

The date etched on her mind was the due date – because, of course, it could have been her own date. "I asked the clinic if they would let me know when her babies were born. Mark and I were in the car when the call came: and they said, she's given birth – they're all fine."

Fertility clinics operate a strict anonymity policy around gamete donation. When a couple choose a sperm or egg donor, they are given basic information: the donor's eye colour, hair colour, educational qualifications and so on. But if the procedure results in a live birth, the family is entitled to more details. "Once we'd had Libby, we were desperate to know more about her genetic father," says Suzanne. "But it turned out to be quite disappointing: he'd only written a few sentences."

She doesn't want to reveal what they are as, she says, it's private information for Libby. But she wishes the donor had told them more about himself so that Libby could learn more about him as she grows up.

As an egg donor, Suzanne was keen to give as much information about herself as possible. "When I was given the chance to write some information for the family who've had the twins, I told them all about myself – what sort of person I am and what I'm interested in," she says. "I told them about Mark, and about how long we waited for a baby. And I told them all about Libby, our gorgeous little daughter."

Libby, of course, is their half-sister. "When they're 18, the twins will have the right to come to look for me, just as Libby will have the chance to look for her genetic father. And if they do, I'd love to meet them, and I'd love them to get to know Libby – I sometimes wonder, do they look like her? Do they have similar personalities?"

Libby may be an only child but she has 10 half-siblings somewhere: apart from the twins, she has another eight brothers or sisters conceived from her genetic father's sperm. "I'd love to think that, one day, she'll be in touch with some of them," says Suzanne.

Mark, meanwhile, says that if Libby wants to go to the US to find her genetic father when she's old enough, he'll be keen to tag along. "I want to shake his hand and say, 'I can't thank you enough, mate,'" he says.

As for Suzanne, she says she has never felt envious of the woman who received her eggs. "I'll always cherish the knowledge that somewhere out there there's another family who only happened because of our heartache."